I have Several 510's, a couple of eGo's, & a Joye 306. Out of all of these the 306 (one of my first e-cigs) is still my favorite.
Whenever you get a new kit you always end up with a couple of those dummy carts with no filling. Well, being the cheapo that I am, I figured out a mod to make use of those and even make your other carts better.

All you need for this is your favorite kind of filling. Poly, Blue foam, or whatever you prefer.

First, take a bent paper clip and shove through the bottom hole of a regular cart and shove the square cart. tray up and out. Next cut a piece of filling (I use blue foam) the same height but double the width. Fold in half sideways and shove in an empty mouthpiece. Not all the way down yet. You want to fill the foam while its still sticking part the way up. Be careful not to overfill! Once you are full you can shove it down to the bottom. It should sit the same height as a regular cart.

Note: If you overfill you will end up with liquid in your mouth. Remember you can always add more if needed.